1. Field of the Invention
This invention relates generally to a method for predicting the peak voltage of a tool or other load at the end of a power transmission cable. In particular, it relates to a method and apparatus for predicting the downhole peak voltage of a wireline tool or tool combination.
2. Description of Related Art
In wireline oil well logging, a number of tools are connected to a power and data transmission cable and lowered into the oil well borehole to obtain measurements of the geophysical properties of the area surrounding the borehole. Well-logging is the measurement of characteristics of different earth formations traversed by a borehole, usually an oil or gas well using one or more measuring instruments or tools. The tools are typically stacked in a tool string, the tool string being attached to a logging cable which supports the tool string, provides power to the tool or tools and provides a communication medium for the transmission of data from the tool or tools to data acquisition and processing equipment on the surface.
Downhole tools receive their power through the transmission cable. An AC power supply is connected on the surface to one end of the cable and the tools are connected to the other end of the cable for conveyance into and out of the borehole (see FIG. 1). FIG. 3 shows schematically how the tools are connected electrically to the uphole power supply. FIG. 4 shows in more detail that each tool rectifies the incoming AC voltage used by the tool to produce a tool DC voltage. For each tool to perform as designed, the tool needs to have its DC voltage V.sub.dc (as shown in FIG. 4) within a specified range. To obtain the specified range for V.sub.dc requires that the peak amplitude of the AC waveform downhole, V.sub.acdh, (FIG. 4) must also be within a specified range. As shown in FIG. 4, V.sub.acdh in turn depends upon the uphole AC voltage V.sub.acuh and the losses on the cable. For the tools to operate properly it is important that the operator be able to set V.sub.acdh very accurately. Setting V.sub.acdh too low prevents the tool from working as designed. Setting V.sub.acdh too high can, in fact, damage the tool.
The difficulty with the waveforms in FIGS. 3 and 4 is that the current waveforms are distorted. The load is time varying and non-linear. The current is only drawn at the peaks of the AC power waveform when the input voltage exceeds the voltage level stored by the capacitor. During conduction, current flows across the high impedance power path causing clipping of the power voltage waveform seen by the tools. The impedance of the high impedance power path and the amount of power supply load and capacitance varies for different tools and cable types and cable lengths. V.sub.dc in FIG. 4 is proportional to the peak value of V.sub.acdh. There is a real need to predict the value of the load's peak voltage waveform remotely from the power driving end in order to apply power without destroying the load.
Currently, oilfield logging engineers go through a procedure in which they temporarily short out the end of the power path, apply some power and then zero out the uphole voltage meter to compensate for the cable impedance. This procedure is called power trim. Since this driving circuitry has no knowledge of the amount of clipping that will occur at the load end of the power path when an actual toolstring is connected, it does not compensate for this time varying load. It compensates as if the load were time invariant.
In other words, this is based on the assumption that EQU V.sub.acdh =V.sub.acuh -(I.sub.wire .multidot.R.sub.wire)
The cable power trim measures R.sub.wire. The circuitry that monitors power at the surface estimates the downhole voltage by first making an RMS measurement then subtracting from it (I.sub.wire .multidot.R.sub.wire). This technique works if sinusoidal waveforms and time invariant loads are assumed. It yields worst case errors of about 30% when these assumptions fail.
The problem with this and other conventional metrics for quantifying sinusoidal voltages or currents are based upon moment calculations (i.e. average, RMS, or mean square). These moments are adequate if the shape of the waveform is well known and consistent (i.e. sinusoidal) because moment calculations are based on the area of the waveform. For these well-behaved waveforms the moment measured can be scaled to yield the measurement we actually want which is the peak value of the waveform. In the wireline logging application the waveforms are distorted, the shapes of the quantity being measured are basically unknown and so the moments cannot be calculated. Scaling a measurement based on these assumptions has proven to be only accurate to .+-.30%.
Another problem that exists in the power trim method is that the measured cable resistance changes as the cable is lowered into the well and heated. This causes a measurement error to occur.
Applications other than wireline logging exist where it is desirable to predict voltage or the power consumption of a load at the end of a transmission cable where direct measurement is not feasible.